Are all MAC addresses unique?

All a MAC address does is identify a particular network device, in the case of an ISP, your modem. They are unique because if you had two NICs in the same network with the same MAC address it you would have severe problems. People have enough problems with duplicate IP addresses on a network. read more

A media access control address (MAC address) of a device is a unique identifier assigned to a network interface controller (NIC) for communications at the data link layer of a network segment. MAC addresses are used as a network address for most IEEE 802 network technologies, including Ethernet and Wi-Fi. read more